Reduced Natural Foods Alter Bottom-Up Pressures on American Black Bears
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Reduced natural foods alter bottom-up pressures on American black bears A Thesis SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY Spencer James Rettler IN PARTIAL FULFULLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE Dr. James Forester © 2018 Spencer James Rettler Abstract Wildlife management is often based on the manipulation of bottom-up and top- down forces. For bear management in North America, food availability (bottom-up) and hunting pressure (top-down) are the primary factors that limit population growth. Whereas seasonal and year-to-year variation in production of fruits and nuts (i.e., primary bear foods) has been widely recognized, long-term trends in natural food production have rarely been reported. Here we compared the current (2015–2017) availability of 18 fruits and nuts, which represent the main foods of American black bears (Ursus americanus) in north-central Minnesota, to what was available during the 1980s. The study area was the same in both time frames, constituting primarily National Forest, where timber harvesting was routine in the 1980s but much less so in recent decades. We hypothesized that forests matured over the 25-year period and consequently produced less fruit for bears. Within each of 12 forest types, we measured the abundance, productivity and biomass (kg/ha) of each fruit-bearing plant species using the same methodology as used in the 1980s. Bootstrapped independent two-sample t-tests identified species-specific changes in food availability between the two sampling periods; generalized linear mixed-effects models were used to quantify changes for groups of fruit-producing species (i.e., summer/fall foods and short/tall shrubs). For all groups of species, the probability of forest stands producing any fruit at all in the recent time period was nearly half (~40%) what it was in the 1980s (~80%). At the landscape scale, we estimated a ~70% decline in biomass availability due to a reduction in young forest types that produce the most fruit, and also a ubiquitous decline in fruit production across most forest types. Our hypothesis that this decline was due to reduced timber harvesting was only partially correct, as we observed the same decline in fruit production within mature forest of the same type and canopy closure, and also along edges of stands with high light penetration. Earlier springs, with potentially greater vulnerability of flowers to frost, and an ongoing invasion of invasive earthworms, which are known to radically affect the soil, may be additional causes for diminished fruit production. Bears will likely need to alter their foraging habits to compensate for lower natural foods as the landscape continues to change. This study demonstrates the complexity of forest management and its unintended effects on wildlife. i Table of Contents Abstract……………………………………………………………………………………i List of Tables…………………………………………………………………………......iii List of Figures…………………………………………..……………….……..................iv Introduction………………………………………………..................................................1 Materials and methods………………………………………………………….………....5 Results…………………………………………………………………………….……...14 Discussion…………………………………………………………………………….….20 Conclusion……………………………………………………………………………….25 Tables…………………………………………………………………………………….27 Figures …………………………………………………………………………………...31 References……………………………………………………………………….……….42 Appendix………………………………………………………………………………....48 ii List of Tables Table 1: Species groupings for Generalized Linear Mixed Effects models……………..27 Table 2: Abundance results for species groupings……………………………...…….....28 Table 3: Productivity results for species groupings……………………………………..29 Table 4: Biomass results for species groupings…………………………………………30 iii List of Figures Figure 1: Map of study area………………………………………………………….….31 Figure 2: Mean species abundance (%) for each forest stand type for the 1980s and 2010s sampling periods…………………………………………………………………..32 Figure 3: Probability of non-zero productivity by sampling period…………………….33 Figure 4: Probability of non-zero productivity in relation to canopy cover…………….34 Figure 5: Year-to-year fluctuations in summer food productivity………………………35 Figure 6: Year-to-year fluctuations in fall food productivity…………………………...36 Figure 7: Mean species biomass by sampling period…………………………………...37 Figure 8: Non-zero biomass in relation to canopy cover………………………………..38 Figure 9: Canopy cover estimates by sampling period………………………………….39 Figure 10: Landscape biomass estimates by sampling period…………………………..40 Figure 11: Landscape biomass estimates by land ownership…………………………...41 iv 1. Introduction Understanding and measuring the effects of top-down and bottom-up pressures on fisheries and wildlife populations has been an area of interest, as well as a struggle, in ecology for several decades (Hunter and Price, 1992). The abundance and types of foods (bottom-up) and predators (top-down) can limit populations, with the relative contribution of these two forces varying by the species of interest and the ecosystem it inhabits (Bowlby and Roff, 1986; Estes, 1995; Frederiksen et al., 2006; Power, 1992). Teasing apart which is the driving force is not always straight forward, particularly when there is within trophic-level competition (Miller, 2008), varying spatial scales (Gripenberg and Roslin, 2007), or unique behavioral components (Laundré et al., 2014). This becomes further complicated when the magnitude of these pressures fluctuates over time (Meserve et al., 2003; Ordiz et al., 2013). For large mammals, humans can become the dominant top-down pressure through hunting (Milner et al., 2007; Ordiz et al., 2013) and vehicular-related mortality (Collins and Kays, 2011). Humans also significantly affect bottom-up pressures via habitat alterations, which can positively or negatively affect wildlife populations (Andrén, 1994; Vospernik and Reimoser, 2008). Hunting pressure and vehicle-related mortality can affect reproductive strategies (Gamelon et al., 2011), genetic diversity (Allendorf et al., 2008), and population size and density (Coffin, 2007). Wildlife management agencies often manipulate both hunting pressure and resource availability to achieve desired population sizes and level of offtake (Ahlering and Faaborg, 2006; Bro et al., 2004). Indeed, it has become commonplace for wildlife agencies to alter habitat to increase food 1 or cover for targeted game species, with the goal of increasing survival, reproduction, and body size –– a strategy first described by Leopold (1933). Oftentimes, though, habitat management has different effects on different species. Climatic factors also influence bottom-up pressures. It is well established that climate change is having substantial effects on forest net primary productivity, phenology, biogeography, and vegetation composition through increased frequency of natural disturbances (Dale et al., 2001; Overpeck et al., 1990), variable temperatures (Allen et al., 2010; Hogg and Bernier, 2005; Pastor and Post, 1988), insect outbreaks (Kurz et al., 2008; Volney and Fleming, 2000), and the spread of pathogens and invasive species (Ayres and Lombardero, 2000; Dukes et al., 2009). In boreal and temperate forests, temperature dictates the beginning and duration of the growing season (Kramer et al., 2000). With warming climate, earlier growing seasons are shifting the timing of resource availability (Cleland et al., 2007; Keyser et al., 2000; Lechowicz, 1995), causing temporally-dependent symbiotic relationships to become disrupted. In forested environments, different combinations of overstory and understory species compositions, tree densities, and canopy closures will provide distributions of food and cover that favor different wildlife species. Here we focus on how these forest attributes can affect the production of food for an omnivorous, but largely frugivorous big game species, the American black bear (Ursus americanus; henceforth black bear). This forest-dependent species was historically over exploited by hunters; however, it is now increasing numerically and expanding geographically across most of its range in the USA, Canada, and northern Mexico (Garshelis et al., 2016). The widespread increase in 2 this species is mainly attributable to purposeful control of top-down forces, specifically reductions in legal and illegal hunting and killing of nuisance animals. Also, in some areas, forest cover has expanded (Hall et al., 2003), generally promoting increased vitality of this species. Food availability affects bear body mass and condition, which greatly influences reproduction (Noyce and Garshelis 1994, Samson and Huot 1995, Costello et al. 2003); this can significantly affect the resilience of the population to hunting (Kontio et al., 1998). Food-related effects occur in two ways: (1) occasional pulses of abundant or sparse foods may affect age of first reproduction (Garshelis et al., 1998; Howe et al., 2012) yielding some particularly strong or weak cohorts (Bridges et al., 2011; Garshelis and Noyce, 2008; McLaughlin et al., 1994); and (2) changes in forest composition may affect overall abundance of foods in the long term, altering reproductive output. A prominent example of the latter occurred during a long-term study of grizzly bears (Ursus arctos) in southern British Columbia, Canada. These bears relied heavily on huckleberries (Vaccinium membranaceum) and the super-abundance of this single key food item boosted grizzly reproduction and density, which for two decades were among the highest in North America